Shang-Fen Ren

Optical anisotropy spectra of GaAs(001) surfaces

We present theoretical studies of the optical anisotropy spectra of GaAs(001) surfaces with different reconstructions at growth temperatures. The Ga‐rich (4×2) missing‐dimer surface, three different phases (α, β, and γ) of the As‐rich (2×4) surface, and an As‐rich c(4×4) surface are considered. Total energy calculations within a nearest‐neighbor tight‐binding model are performed to determine the surface geometry for each reconstruction considered. The difference between reflectance spectra with [110] and [110] polarization for these surfaces is analyzed and compared to available data.

Study of reconstruction at interfaces of CdSe/ZnTe superlattices by total energy calculations

ZnTe/CdSe superlattices are II–VI heterostructures with a lattice mismatch less than 0.3%, so strain effects are expected to be negligible for ideal structures except for the two interface layers that consist of ZnSe and CdTe. Recent experiments indicated the exchange of whole atomic layers across the interfaces in ZnTe/CdSe superlattices despite the fact that more atomic layers near the interface would become strained. To understand such exchanges, first‐principles pseudopotential studies of several different interface structures were performed. Total energies, band offsets, and band structures of superlattices with different atomic rearrangements at interfaces are studied and compared. Our results indicate that the exchange of cation atomic layers across an anion atomic layer in these superlattices is energetically favorable.

Phonons in semiconductor superlattices

Abstract We present theoretical studies of phonon modes in semiconductor superlattices in a rigid-ion model. We examine the dispersion curves for phonons propagating in any directions, taking into account the long-range Coulomb interaction. We introduce an alternative approach which avoids direct computation of the Coulomb interaction in superlattices. Our calculations indicate that several superlattice optical phonon branches can have different frequencies when → k approaches zero from different directions. Such anisotropic splitting was observed experimentally and was interpreted in a macroscopic model involving layers of different dielectric constants. The predictions of our microscopic model are in qualitative agreement with those of the macroscopic model.

Phonon dispersion curves of GaAsAlAs superlattices

Abstract The phonon properties of (GaAs)m(AlAs)n superlattices are studied with an eleven-parameter rigid-ion model. Short-range interactions up to the second neighbours are included, and the long-range Coulomb interaction is calculated exactly. Modes propagating both normal (k// = 0) oblique (k// ≡ 0) to the interfaces are studied. Anisotropy of zone center optical phonons is examined. The theoretical results are compared with the existing experimental data with favorable agreement.